Here we study how the mains voltage is controlled to 12V and 5V using just a single IC LR645G and some other supportive ordinary active semiconductors.


In one of my earlier posts I provided a similar circuit but it utilized a high voltage capacitor for dropping the mains voltage to lower usable levels.

Thanks to Supertex ic. for providing us with this wonderful little chip LR645G, which single handedly controls any voltage between 24 and 270 V AC and produces DC voltages below 15 volts at the output, which becomes ideally suitable for operating sensitive, compact electronic circuits.

The best part of the circuit is that it does not incorporate any bulky of heavy components like a transformer or non polar high voltage capacitors.

Though we all know the simple way of constructing transformerless power supply units using high voltage capacitors, these high voltage capacitors have one big drawback.

At switch ON, these caps allow high surge inputs to pass through them and also intermediate transients become unstoppable with these devices.

The drawback can cause havoc with any electronic circuit that may be connected to  such power supply configurations.

Using LR645G

Using LR645G the above threat becomes absolutely nullified.  The maximum current available from this device is quite low, around 3 mA, however that's never a problem, because the current can be shot up to 150 mA, through a simple addition of an  fet DN2540N5 in the circuit.

The figure shown above is a classic circuit set up of a 12V and 5V transformer less power supply circuit which can provide outputs of 15 volts and 5 volts.

15 volts is available just at the junction of the output of LR645 and the input of Ic 7805.
If the 5 volt option is not required, the configuration around the 5 volt regulator can be just eliminated, which makes the circuit yet simpler and compact.

How the Circuits Works

In short the circuit diagram may be understood in the following manner:

  • The high voltage AC mains is rectified by the bridge configuration using four diodes at the input.

  • The rectified voltage is smoothed by the filter capacitor introduced just after the bridge network.

  • The rectified, filtered high voltage is fed to the IC LR645LG, which effectively reduces the voltage to 15 volts at 3 mA.

  • The FET pulls the 3 mA current output to 150 mA and feds it to the next stage which incorporates the 5 volt regulator stage.

However one big drawback of eliminating the use of a transformer is the DANGER of high voltage shock that actively hangs with all the naked points of the circuit.

Therefore extreme caution must be exercised while building and testing this circuit and other attached circuits.

Parts List

Diodes - 1N4007

Input Capacitor - 4.7uF/400V,

Output Capacitors are 1uF/25V

ICs are LR645LG and 7805,

FET - DN2540N5

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